Airflow is the reading that most often explains a comfort complaint, a frozen coil or a system that never reaches its rated capacity, and it is the reading most often skipped because it takes a few minutes and a little arithmetic. This page is for the technician or estimator in a US HVAC contracting business who needs the HVAC cfm calculation in two forms: the design starting point from tonnage, and the field measurement on a running system. The airflow-per-ton figures below are the Air Conditioning Contractors of America's, linked as the source; the trunk line calculator on this site sizes the duct that has to carry the result.
How to get cfm from tonnage: the starting point
The rule of thumb the trade starts from is 400 CFM per ton of cooling, and the ACCA's own guidance is careful to call it a typical figure that many factors change. In hot, humid climates a lower airflow such as 350 CFM per ton gives the coil more dehumidification; in warm, dry climates a higher airflow such as 450 CFM per ton lets the same system deliver more sensible capacity. The manufacturer's data for the specific equipment overrides all three. Use the rule to size ducts and to sanity check a reading; never to commission a system.
The cfm formula HVAC technicians use in the field
Airflow can be calculated from the sensible heat the coil is moving: measure the temperature drop across the coil and the sensible capacity the equipment is delivering at the measured conditions, and the airflow follows from the sensible heat formula, using the air density constant for the conditions. The method is only as good as the capacity figure, which comes from the manufacturer's expanded performance data at the measured indoor and outdoor conditions, not from the nominal tonnage.
How to test cfm directly
Total external static pressure measured across the equipment, read against the manufacturer's blower table for the fan speed in use, gives the airflow the blower is delivering. It is the fastest field method and the most common. A flow hood at the registers sums the supply airflow room by room and shows where the duct system is losing it. A pitot traverse in the trunk is the most accurate and the least used. Record the method with the reading, because the methods do not agree exactly.
What the reading tells you
Airflow well below the design figure with high static means the duct system or the filter is restricting; look at return size, filter type and dirty coils. Low airflow with low static means the blower is not delivering; look at speed taps, belts and the wheel. Airflow far above the design figure means a coil that will not dehumidify. The reading, recorded against the unit on the maintenance or installation checklist, is the diagnosis's evidence.
From the airflow to the duct
Once the design airflow is known, the trunk and branch ducts have to carry it at an acceptable velocity and friction rate. The trunk line calculator on this site sizes a rectangular or round trunk from the airflow and the friction rate you enter, which is the next step in a replacement estimate when the existing ducts are suspect.
Questions people ask about hvac cfm calculation
Is 400 CFM per ton always right?
No. It is the typical starting point, and the ACCA's guidance names climate as the reason it moves between roughly 350 and 450. The manufacturer's data for the installed equipment decides.
What is the quickest way to test cfm on a service call?
Total external static pressure against the blower table. It takes minutes with a manometer and tells you at once whether the duct system is the problem.
Should HVAC cfm calculation be on the installation checklist?
Yes, as a commissioning reading with the method noted. It is the reading that most often defends the install later.